Prosthetic heart valves are well known in the art. The advent of the prosthetic heart valve has provided many patients with both improved quality of life and increased longevity. There are a variety of known conventional prosthetic heart valves available for use by the cardiac surgeon. Heart valves can be classified as mechanical or tissue (biological) valves. Examples of tissue valves include porcine heart valves, which are harvested intact and subsequently processed by glutaraldehyde in order to preserve the tissue valve. Other known tissue valves include those in which the leaflets are made from bovine pericardium, equine pericardium, and human pericardium.
Mechanical heart valves typically have a frame or structure containing the moveable elements (i.e. leaflets or occluder) that reproduce the function of the native or autologous leaflets of the heart valve. The main components of the valve structure, typically, consists of a valve ring which houses the leaflets and a sewing ring, which is the site of attachment of the valve to the native annulus. The leaflets of the mechanical valves may be made of conventional, biocompatible materials such as pyrolytic carbon, and are mounted in the valve ring by hinge points allowing hinged movement of the leaflets. The sewing ring of the valve provides an area for sutures or other means of fixation to mechanically attach the prosthetic valve to the annulus of the heart. The sewing ring is typically a synthetic polymeric woven structure that extends outward from the periphery of the prosthetic heart valve.
The primary function of a prosthetic heart valve is to act as a check valve, opening to permit antegrade blood flow and closing to prevent retrograde flow, about one hundred thousand times a day. The moveable elements move in response to a threshold pressure gradient in a direction allowing flow through the valve, while closing and preventing reverse flow below the threshold pressure gradient. The prosthetic heart valve simulates the function of a natural heart valve.
A heart valve replacement surgical procedure is typically performed in the following, conventional manner: The patient is placed on Cardiopulmonary Bypass (CPB) and their heart is arrested with the use of a cardioplegia solution. The flow of oxygenated blood is maintained throughout the systemic circulation, excluding the heart, through the use of conventional roller pumps on the CPB unit maintaining viability of the tissue. Surgical exposure of the diseased valve is then performed. The leaflets of the valve may be removed, as with the aortic valve, or secured back, as with the mitral valve. The native valve is then debrided of all visible signs of calcium. Sutures are placed around the annulus of the valve in either an everted or non-everted configuration, and the surgical needles that are mounted to the individual suture legs are secured to a suture management system. Once all the sutures have been placed around the annulus, the needles are then passed through the sewing ring of the valve and the valve is parachuted down to the annulus. The legs of the individual sutures are knotted with typically six to seven knots to ensure fixation of the valve.
Prosthetic heart valves go through extensive testing by the valve manufacturer to ensure the durability of the device, the fluid flow characteristics, and the leakage through the valve orifice with the leaflets in the closed position. Failure of the prosthetic heart valves in vivo can have catastrophic results. However, the manufacturer's focus of the tests is with respect to the valve functionality and not the fixation or attachment of the valve to the annulus of the heart of the patient.
One of the critical parameters of a successful valve replacement procedure is attaching the valve to the native annulus and creating a seal between the valve sewing ring and annular tissue that eliminates the leakage of blood between the two. This type of leakage is referred to as paravalvular leakage. Paravalvular leakage is defined as the flow of blood from one side of the valve to the other, while the valve is closed through regions other then through the orifice of the valve (i.e., through the sewing ring, under the sewing ring, and alongside the suture). The assessment of paravalvular leakage has historically been performed either in vitro in a bench top pulse duplicator system or in vivo in acute or chronic animal studies, using indirect methods of analysis, such as color flow Doppler ultrasound imaging or the transvalvular pressure tracings to assess a valve fixation device or technique. The effects of paravalvular leakage upon the post operative valve transplant patient include: hemolysis, hemodynamic instability, dehiscence, or valvular dysfunction.
There are disadvantages associated with these historical methods. Ultrasound color flow Doppler imaging algorithms yield a high degree of random and systematic error when assessing a 3-dimensional structure with multidirectional flow regions. Transvalvular pressure measurements, throughout the cardiac cycle, have also been used to attempt to evaluate the degree of paravalvular leakage, however, isolating alterations in the transvalvular pressure tracing to a paravalvular leakage is inaccurate and unquantifiable. The methods used to date involve indirect measurements of leakage from regions of the valve structure that is fully contained within the natural flow field (i.e. aorta or left atrium). None of these techniques provide a true quantifiable means of the absolute measurement of paravalvular leakage.
While existing methods of valve testing apply only to the evaluation of the valve leaflet function, there is an unmet and pressing need for a novel apparatus and in vitro method for evaluation of the integrity of valve fixation and the measurement of paravalvular leakage.